Conventionally isoprene is produced by extraction from pyrolysis gasoline, which is a byproduct of steamcracking of naphtha. The yield is typically very low, of the order of 1-3% of the produced ethylene. Hence it is difficult to justify this capital-intensive technology for only a small production capacity of isoprene. The process to isolate isoprene from pyrolysis gasoline consist first in the removal of cyclopentadiene by dimerisation and distillation. Next the pipirylenes are separated by superfractionation. The last steps consist in an extractive distillation using a solvent. Moreover the quality of isoprene obtained from pyrolysis gasoline is hard to guarantee as the specifications with respect to cyclopentadiene and pipirylenes are very severe and these compounds are plentiful present in the same pyrolysis gasoline. As pyrolysis gasoline contains only small amounts of isoprene (10-20%), a lot of byproducts (dicyclopentadiene and pipirylene) are produced according to the same laborious manner while their market value is not necessary in line with the evolution of the market value of isoprene.
Recently, there is a tendency to shift to lighter feedstock for steamcracking feeding. Most new steamcrackers are using ethane as feedstock that does not produce pyrolysis gasoline as byproduct. Also many naphtha-based steamcrackers are shifting to lighter feedstock because of its abundant availability and competitive advantage.
Other routes to produce isoprene are the isolation of isoamylenes from refinery and petrochemical cuts and perform a dehydrogenation into isoprene. This process is typically done over iron oxide catalyst promoted with potassium compounds at temperatures above 600° C. in presence of water steam and reduced pressure. As this reaction is limited by a thermodynamic equilibrium, only partial conversions can be obtained.
Isoprene can also be produced from isopentane by a double dehydrogenation.
In still another process, isoprene is produced by a two-step process. In the first step iso-butene, tertiary-butanol, di-t-butyl ether, methyl-t-butyl ether or ethyl-t-butyl ether is condensed with two molecules of formaldehyde to form dimethyloxirane. The dimethyloxirane is separated and purified. In the second step the dimethyloxirane is decomposed under appropriate conditions into isoprene and one molecule of formaldehyde. An improvement on the latter two-step process is a one-step process, in which iso-butene, tertiary-butanol, di-t-butyl ether, methyl-t-butyl ether or ethyl-t-butyl ether is directly reacted with formaldehyde into isoprene.
The U.S. Pat. No. 4,511,751 describes a process for producing isoprene in good yield. The process is characterized in that iso-butene and/or tertiary butanol and a formaldehyde source are fed, together with water, into an acidic aqueous solution continuously or intermittently while maintaining the reaction pressure in an adequate range and at the same time distilling off the product isoprene and unreacted starting materials, together with water, from the reaction zone.
The U.S. Pat. No. 4,593,145 describes a process for producing isoprene, characterized in that an alkyl-t-butyl ether and a formaldehyde source are fed, together with water, into an acidic aqueous solution continuously or intermittently while maintaining the reaction pressure in an adequate range and at the same time distilling off the product isoprene, unreacted starting materials, iso-butene and tertiary butanol, together with water, from the reaction zone.
EP106323 describes a process for producing isoprene by reacting iso-butene and/or tertiary butanol and/or an alkyl tertiary butyl ether which gives iso-butene and/or tertiary butanol under the reaction conditions with formaldehyde in an acidic aqueous solution, under such conditions (a) that the acidic aqueous solution is present in the reaction zone, (b) that iso-butene and/or tertiary butanol and/or the alkyl tertiary butyl ether, a formaldehyde source and water are fed to said reaction zone continuously or intermittently, and (c) that isoprene, water, unreacted starting materials and other low-boiling components are distilled off from said reaction zone, wherein a glycol ether is added, in an amount of 5 to 15 percent by weight, to the acid aqueous solution. It is specified that the presence of a solvent in the reactor improves the solubility of iso-butene in the aqueous phase and hence the contact with the acid catalyst that is substantially dissolved in the aqueous solution.
EP 1 614 671 A1 describes process for producing isoprene, which includes continuously or intermittently supplying iso-butene and/or t-butanol, formaldehyde and water into an acidic aqueous solution, and reacting the reaction mixture while distilling away a mixture containing produced isoprene, water, unreacted starting materials and other low boiling point components from this reaction mixture to the outside of the reaction system, wherein the reaction is carried out while controlling the concentration of high boiling point byproducts, which is produced and accumulated in the reaction mixture, to fall within the range of 0.5-40 mass %.
EP 2 157 072 A1 describes a method to obtain isoprene by way of liquid-phase interaction between trimethyl carbinol (also known as t-butanol, or its water solutions) and formaldehyde (or its source substances) in the presence of acidic catalyzer water solution; this can be made in one or several contacting stages, with use (at the final contacting stage) of separation reactor containing a heat supply zone, a reaction zone and a separation zone, with reaction products and water taken, out of the separation zone, in the form of a vapor flow to be subsequently cooled down, condensed and separated and with liquid flow of the catalyzer water solution put out for extraction and, after this, put back into the heating zone. As it goes from the reaction zone into the separation zone, the reactive flow is throttled. In the reaction zone, temperature is maintained at the level of 140-180° C., while pressure is 8-25 atmospheres; in the separation zone, pressure is 1.2-9.5 atmospheres. The separation reactor contains two or three separation zones. The balance quantity of water is put out of catalyzer water solution, which is circulating along the circuit, by way of its evaporation as the flow is throttled into the separation zone (zones) during regulation of the quantity of the circulating liquid phase in the interval of 0.2-6.0 parts of the total reaction zone area.
US 2010 0216958 A1 relates, in one embodiment, to a method of preparing butadiene comprising (a) providing an alcohol mixture comprising one or more butanols; (b) contacting the alcohol mixture with a dehydration catalyst, thereby forming an olefin mixture comprising one or more linear butenes and isobutene; (c) contacting the olefin mixture of step (b) with a dehydrogenation catalyst, thereby forming a di-olefin mixture comprising butadiene and isobutene; and (d) isolating butadiene from the di-olefin mixture of (c).
In another embodiment, it relates to a method of preparing isoprene comprising (a) providing an olefin mixture comprising one or more pentenes, with the proviso that at least a portion of the olefin mixture comprises one or more methylbutenes; (b) contacting the olefin mixture of (a) with a dehydrogenation catalyst, thereby forming a mixture comprising isoprene; and (c) isolating isoprene from the mixture of (b).In still another embodiment, it relates to a method of preparing monomers, comprising: (a) providing an olefin mixture comprising one or more linear butenes and isobutene; (b) contacting the olefin mixture of step (a) with a dehydrogenation catalyst, thereby forming a di-olefin mixture comprising butadiene and isobutene; (c) isolating isobutene from the mixture of step (b); and (d1)) converting the isobutene to methyl t-butyl ether, ethyl t-butyl ether, isooctane, methacrolein, methyl methacrylate, butyl rubber, butylated hydroxytoluene, or butylated hydroxyanisole.
In still other embodiments, it relates to methods for preparing isobutene or isoprene as described herein, wherein the olefin mixture is prepared by dehydration of a renewable alcohol mixture comprising one or more renewable C4 or C5 alcohols.
Iso-butanol (2-methyl-1-propanol) has historically found limited applications and its use resembles that of 1-butanol. It has been used as solvent, diluents, wetting agent, cleaner additive and as additive for inks and polymers. Recently, iso-butanol has gained interest as fuel or fuel component as it exhibits a high octane number (Blend Octane R+M/2 is 102-103) and a low vapor pressure (RVP is 3.8-5.2 psi).
Iso-butanol is often considered as a byproduct of the industrial production of 1-butanol (Ullmann's encyclopedia of industrial chemistry, 6th edition, 2002). It is produced from propylene via hydroformylation in the oxo-process (Rh-based catalyst) or via carbonylation in the Reppe-process (Co-based catalyst). Hydroformylation or carbonylation makes n-butanal and iso-butanal in ratios going from 92/8 to 75/25. To obtain iso-butanol, the iso-butanal is hydrogenated over a metal catalyst. Iso-butanol can also be produced from synthesis gas (mixture of CO, H2 and CO2) by a process similar to Fischer-Tropsch, resulting in a mixture of higher alcohols, although often a preferential formation of iso-butanol occurs (Applied Catalysis A, general, 186, p. 407, 1999 and Chemiker Zeitung, 106, p. 249, 1982). Still another route to obtain iso-butanol, is the base-catalysed Guerbet condensation of methanol with ethanol and/or propanol (J. of Molecular Catalysis A: Chemical 200, 137, 2003 and Applied Biochemistry and Biotechnology, 113-116, p. 913, 2004).
Recently, new biochemical routes have been developed to produce selectively iso-butanol from carbohydrates. The new strategy uses the highly active amino acid biosynthetic pathway of microorganisms and diverts its 2-keto acid intermediates for alcohol synthesis. 2-Keto acids are intermediates in amino acid biosynthesis pathways. These metabolites can be converted to aldehydes by 2-keto-acid decarboxylases (KDCs) and then to alcohols by alcohol dehydrogenases (ADHs). Two non-native steps are required to produce alcohols by shunting intermediates from amino acid biosynthesis pathways to alcohol production (Nature, 451, p. 86, 2008 and US patent 2008/0261230). Recombinant microorganisms are required to enhance the flux of carbon towards the synthesis of 2-keto-acids. In the valine biosynthesis 2-ketoisovalerate is on intermediate. Glycolyse of carbohydrates results in pyruvate that is converted into acetolactate by acetolactate synthase. 2,4-dihydroxyisovalerate is formed out of acetolactate, catalysed by isomeroreductase. A dehydratase converts the 2,4-dihydroxyisovalerate into 2-keto-isovalerate. In the next step, a keto acid decarboxylase makes isobutyraldehyde from 2-keto-isovalerate. The last step is the hydrogenation of isobutyraldehyde by a dehydrogenase into iso-butanol.
Of the described routes towards iso-butanol above, the Guerbet condensation, the synthesis gas hydrogenation and the 2-keto acid pathway from carbohydrates are routes that can use biomass as primary feedstock. Gasification of biomass results in synthesis gas that can be converted into methanol or directly into iso-butanol. Ethanol is already at very large scale produced by fermentation of carbohydrates or via direct fermentation of synthesis gas into ethanol. So methanol and ethanol resourced from biomass can be further condensed to iso-butanol. The direct 2-keto acid pathway can produce iso-butanol from carbohydrates that are isolated from biomass. Simple carbohydrates can be obtained from plants like sugar cane, sugar beet. More complex carbohydrates can be obtained from plants like maize, wheat and other grain bearing plants. Even more complex carbohydrates can be isolated from substantially any biomass, through unlocking of cellulose and hemicellulose from lignocelluloses.
It is the object of the present invention to use of iso-butanol for the production of isoprene by condensation with formaldehyde. Without willing to be bound to any theory, it is believed that the t-butyl-carbocation is the reactive specie that attacks formaldehyde and that its presence in the aqueous solution where resides also the acid catalyst and the formaldehyde is essential for high reaction rates for the selective condensation reaction. The decomposition of iso-butanol is significantly slower than that of t-butanol under the reaction conditions and as a consequence iso-butanol will serve as efficient solvent that improves the solubility of iso-butene and enhances the presence of t-butyl-carbocations in the aqueous phase. t-Butanol tends to dehydrate too fast so that most of the iso-butene escapes from the aqueous reaction medium and hence a lot of recycling is required.
The following reactions occur under the reaction conditions:
Dehydration:

Condensation:

It is the object of the present invention to produce isoprene by reacting formaldehyde with an iso-butene producing alcohol comprising isobutanol.